Question: The arithmetic sequence $(a_i)$ is defined by the formula: $a_1 = -1$ $a_i = a_{i-1} + 2$ What is $a_{15}$, the fifteenth term in the sequence?
Solution: From the given formula, we can see that the first term of the sequence is $-1$ and the common difference is $2$ To find the fifteenth term, we can rewrite the given recurrence as an explicit formula. The general form for an arithmetic sequence is $a_i = a_1 + d(i - 1)$ . In this case, we have $a_i = -1 + 2(i - 1)$ To find $a_{15}$ , we can simply substitute $i = 15$ into the our formula. Therefore, the fifteenth term is equal to $a_{15} = -1 + 2 (15 - 1) = 27$.